1. Field of the Invention
The present invention relates to an electrophotographic image forming apparatus or a process cartridge including a unit that applies a lubricant to the surface of an image carrier, and a toner used in the image forming apparatus or the process cartridge.
2. Description of the Related Art
An image forming apparatus using an electrophotographic process has a photoconductor as an image carrier, and the surface of the photoconductor is electrostatically charged by discharge and the charged surface of the photoconductor is exposed to form an electrostatic latent image, and a toner is supplied to the electrostatic latent image to make the image visible, and the visible image formed on the surface of the photoconductor is transferred to the surface of transfer paper and then fixed and discharged. Some of the toner remains on the surface of the photoconductor after the visible image is transferred, and therefore, for preventing the remaining toner from adversely affecting the next image formation, the surface of the photoconductor is cleaned by a cleaning unit and then subjected to the subsequent image forming process. A generally known cleaning unit having a cleaning blade includes an elastomer, such as a rubber, which is abutted against the surface of the photoconductor to remove deposits including the remaining toner.
There are increasing demands for high-quality images, and particularly, for achieving the formation of high definition color images, a toner having a smaller particle diameter or a spherical shape has been developed. The toner having a smaller particle diameter can improve dot reproductivity, and the toner having a spherical shape can improve the development property and the transfer property. The toner having a smaller particle diameter or a spherical shape is very difficult to produce by a conventional grinding method, and hence a polymerized toner produced by a suspension polymerization method, an emulsion polymerization method, or a dispersion polymerization method is employed.
However, the use of the toner having a spherical shape or a smaller particle diameter causes some problems in cleaning of the photoconductor after the image is formed. One problem is that the toner having a spherical shape or a smaller particle diameter is difficult to remove by a blade cleaning method generally employed. A cleaning blade, which is rubbed against the surface of the photoconductor to remove the toner from the photoconductor, is deformed at its edge by the frictional resistance of the photoconductor, causing a very small gap between the photoconductor and the cleaning blade; The smaller the particle diameter of the toner, the more likely the toner is brought into the gap. When the toner brought into the gap is spherical or substantially spherical, the frictional force by rolling is smaller and causes the toner to roll through the gap between the photoconductor and the cleaning blade, so that the toner passes under the cleaning blade. When a large amount of the toner passes under the cleaning blade, the cleaning for the photoconductor becomes unsatisfactory, leading to a defective image suffering from, for example, background fogging.
Another problem is that a lubricating agent or a superplasticizer contained in the toner, which passes under the cleaning blade and then remains on the surface of the photoconductor, forms a film material sticking to the surface of the photoconductor, i.e., so-called filming occurs. The occurrence of filming causes a defective image with, for example, unprinted points in the solid area of image.
For improving the cleaning property for the surface of the photoconductor when using the toner having a spherical shape or a smaller particle diameter, a method has been proposed in which a lubricant includes, for example, a metal salt of a fatty acid is applied to the surface of the photoconductor to form a thin film, lowering the friction coefficient of the surface of the photoconductor, for example, as disclosed in Japanese Patent Application Laid-open No. 2002-287567. It has already been found that, when the friction coefficient of the surface of the photoconductor is low, the adhesive force between the photoconductor and the toner is small and hence cleaning by the blade becomes effective, thus preventing the occurrence of filming.
When the friction coefficient of the photoconductor is not satisfactorily lowered, the toner is likely to pass through the cleaning unit to cause a cleaning failure or filming is likely to occur on the surface of the photoconductor. When filming by the lubricating agent or superplasticizer contained in the toner occurs, the resultant film material is difficult to remove by the cleaning blade.
Especially, the developer in the developing unit includes large amounts of a lubricating agent, a superplasticizer, and the like derived from the toner, and hence filming is likely to occur immediately on the photoconductor coated with the lubricant only with a small coating amount. Therefore, it is necessary to constantly apply the lubricant to the photoconductor in an appropriate coating weight during the life of the photosensitive unit.
On the other hand, when the friction coefficient of the surface of the photoconductor is too small, other problems arise. One of the problems is such that the adhesive force between the photoconductor and the toner is lowered and therefore a satisfactory amount of the toner is not transferred from a development roller to the electrostatic latent image formed on the photoconductor in the development step, thus causing unprinted portions in the image or a low-density image, for example, as disclosed in Japanese Patent Application Laid-open Nos. 2002-287567 and 2002-207397.
Japanese Patent Application Laid-open No. 2000-75752 includes a description that a shape factor SF1 is 100 to 150, a cleaning unit has an elastic blade abutted at a linear pressure of 55 g/cm to 105 g/cm, and a lubricant application mechanism applies at least 0.4 gram or more of a lubricant until a rotation distance of an image carrier to be coated becomes 525 meters. When the linear pressure of the cleaning blade is high, the blade disadvantageously scrapes the lubricant from the image carrier to lower the application efficiency, so that the photoconductor CTL layer is considerably quick to wear. By surely applying a lubricant in an appropriate coating weight constantly during the life of the photosensitive unit, excellent images can be provided stably.